The present invention generally relates to coatings for components exposed to high temperatures, such as the hostile thermal environment of a gas turbine engine. More particularly, this invention is directed to thermal barrier coatings having modulated columnar microstructures that increase the impact resistance of the coatings.
Components within the hot gas path of gas turbine engines are often protected by a thermal barrier coating (TBC). TBC's are typically formed of ceramic materials deposited by plasma spraying, flame spraying and physical vapor deposition (PVD) techniques. Various ceramic materials have been proposed for TBC's, the most widely used being zirconia (ZrO2) partially or fully stabilized by yttria (Y2O3), magnesia (MgO), or ceria (CeO2) to yield a tetragonal microstructure that resists phase changes. Yttria-stabilized zirconia (YSZ), and particularly YSZ containing about six to eight weight percent yttria (6-8% YSZ), has been the most widely used TBC material due at least in part to its high temperature capability, low thermal conductivity, and relative ease of deposition by plasma spraying, flame spraying and PVD techniques. To promote adhesion of TBC to metallic substrates, such as superalloys used in gas turbine engine applications, a metallic bond coat is usually deposited on the substrate before applying the TBC. Bond coats are typically an aluminum-rich composition, such as an overlay coating of an MCrAlX alloy or a diffusion coating such as a diffusion aluminide or a diffusion platinum aluminide. As a result of oxidation, bond coats formed of these compositions develop an aluminum oxide (alumina) scale that chemically bonds the TBC to the bond coat and the underlying substrate.
Spraying techniques deposit TBC material in the form of molten “splats,” resulting in a TBC characterized by a degree of inhomogeneity and porosity. TBC's employed in the highest temperature regions of gas turbine engines are most often deposited by PVD, particularly electron-beam PVD (EBPVD), which yields a strain-tolerant columnar grain structure that is able to expand and contract without causing damaging stresses that lead to spallation. Similar columnar microstructures can be produced using other atomic and molecular vapor processes, such as sputtering (e.g., high and low pressure, standard or collimated plume), ion plasma deposition, and all forms of melting and evaporation deposition processes (e.g., laser melting, etc.).
In addition to being well adhered and having low thermal conductivities, TBC's on gas turbine engine components are required to withstand damage from impact by hard particles of varying sizes that are generated upstream in the engine or enter the high velocity gas stream through the air intake of a gas turbine engine. The result of impingement can be erosive wear (generally from smaller particles) or impact spallation from larger particles. Impact spallation is a primary issue at and near the leading edge of gas turbine engine blades and vanes, where the likelihood of damage from impact spallation is sufficiently high that the thermal protection of TBC deposited on a leading edge of a blade or vane is often not taken into consideration when designing the blade or vane. As a consequence, greater amounts of cooling air are necessary to maintain an acceptable blade/vane surface temperature.
FIG. 1 depicts one of the mechanisms of damage caused by a particle 20 impacting a TBC 14 adhered with a bond coat 12 to a substrate 10. The TBC 14 is represented as having a columnar grain structure of the type described above. As such, the TBC 14 comprises individual columns 16 separated by gaps 18, resulting in a porous microstructure. An interface 26 exists between the TBC 14 and bond coat 12, where adhesion between the TBC 14 and bond coat 12 is promoted by alumina scale (not shown). The impacting particle 20 generates stress waves 22 in the outer surface region of the impacted columns 16. The stress waves 22 travel downward through the impacted columns 16, arriving at the interface 26 as reflected stress waves 24. The stresses generated by the stress waves 22 and 24 are compressive in the first few columns 16, but become tensile in succeeding columns 16 (as viewed in FIG. 1, those columns 16 to the right of the impacted columns 16). When these tensile stresses reach the interface 26 between the TBC 14 and bond coat 12, separation of the TBC 14 at the interface 26 can occur depending on the magnitude of the tensile stresses. In such an event, the TBC 14 completely separates (spalls) from the bond coat 12.
Commonly-assigned U.S. Pat. No. 6,352,788 to Bruce teaches that YSZ containing about one up to less than six weight percent yttria in combination with magnesia and/or hafnia exhibits improved impact resistance. In addition, commonly-assigned U.S. Pat. No. 7,060,365 to Bruce shows that small additions of lanthana, neodymia and/or tantala to zirconia partially stabilized by about four weight percent yttria (4% YSZ) can improve the impact resistance of 4% YSZ. It would be desirable if further improvements in impact resistance could be obtained.